Vehicle air conditioning systems (broadly defined to include both heating and cooling the air) are often referred to by the shorthand acronym of "HVAC" system. The heart of such a system is a box shaped housing containing an evaporator and heater, which are spaced apart, with inner faces that face one another and outer faces that face away from one another. Fan forced air flow is selectively directed through the two heat exchangers, cold and hot, to attain a final, mixed air stream of a desired temperature and flow rate. In essentially all commercially available systems, the evaporator is located upstream of the heater, and is the physically larger of the two heat exchangers, so that all of the forced air stream passes through it initially. However, the evaporator can be deliberately turned off, so the fact that all air passes through it all the time does not jeopardize the ability to control final temperature. The heater, however, typically operates all the time, so that the system must be able to route or block air selectively through the heater, in order to achieve a desired final, mixed temperature. Older mechanisms for blocking or unblocking the air flow through the heater used a swinging flapper door located in the space between the evaporator and heater, which would admit more or less air through the heater depending on its angular position. The final temperature, mixed air stream would finishes downstream of the beater. Such systems obviously require enough space between the evaporator and heater for the door to swing, limiting how compact the entire system can be made. In addition, swinging door systems tend to lack linearity. That is, they tend to be all on, or all off, but are far less adept at attaining. mid range settings.
More recent designs, attempting to attain both improved packaging and better linearity, have incorporated a rolling film belt to selectively block or unblock air flow through the heater. An example may be seen in U.S. Pat. No. 5,653,630. The design disclosed there uses a single belt (temperature belt) wrapping around the entire inner face of the heater, and which also extends up beyond the heater inner face and partially over, but only partially over, the inner face of the evaporator. The portion of the inner face of 10 the larger evaporator not covered by the single film temperature belt is selectively blocked or unblocked by a swinging door of conventional design. Air that has passed the evaporator is let through, or by passed around, the heater by a combined action of the moving belt and the swinging door, to mix together downstream of the heater. An entirely separate belt (mode belt) moves independently to admit the mixed, final temperature air into the passenger compartment.
The single belt temperature control disclosed, and any single belt design, suffers from an inevitable shortcoming, however. A single belt, as it moves, inherently shifts solid areas to locations where open areas of the belt previously were, and vice versa. Open and blocked areas are not independently achievable, in other words, which means that not every desired combination of final temperature and air flow rate can achieved. A temperature change created by allowing more or less air through the heater core inevitably affects total final air flow rate, as well. The extra by pass door in the design disclosed in U.S. Pat. No. 5,653,630 noted above which needs its own actuator and swinging room which negates much of the advantage of using a film belt in the first instance.